Luther and the Lutheran Confessions (explaining “thou shalt not commit adultery”) on the problems of forbidding marriage for the clergy, and how this manifests itself in sexual sins:
Priests, monks, and nuns resist God’s order and commandment, inasmuch as they despise and forbid matrimony, and presume and vow to maintain perpetual chastity, and, besides, deceive the simple-minded with lying words and appearances [impostures]. For no one has so little love and inclination to chastity as just those who because of great sanctity avoid marriage, and either indulge in open and shameless prostitution, or secretly do even worse, so that one dare not speak of it, as has, alas! been learned too fully.
And, in short, even though they abstain from the act, their hearts are so full of unchaste thoughts and evil lusts that there is a continual burning and secret suffering, which can be avoided in the married life. Therefore all vows of chastity out of the married state are condemned by this commandment, and free permission is granted, yea, even the command is given, to all poor ensnared consciences which have been deceived by their monastic vows to abandon the unchaste state and enter the married life, considering that even if the monastic life were godly, it would nevertheless not be in their power to maintain chastity, and if they remain in it, they must only sin more and more against this commandment.